Enhancement Idea: Split the instructions by submodule

[sp4ghet]

Right now, the instructions are all in one markdown document: instructions.md

However, the previous version had instructions split by shell/frame/motherboard. I suggest we do the same and split the documentation by submodule based on the following reasons:

1. It's easier to follow. Many of the submodules can be assembled separately and in parallel but this is not immediately obvious. By breaking the instructions into smaller pieces we can indicate more concisely that the separate submodules can be assembled in parallel.
2. Markdown Editors/Previewers have a difficult time loading large monotonic markdown files. Atom, a common text editor has a markdown previewer built-in, but it crashes when trying to preview the current instructions.md. The github web client is also very slow to render the images making it difficult to stay in one place reading the instructions because the newly loaded images force the scroll position to move. By splitting the file into smaller pieces we can solve this problem as well.

I'm working on a fix and will submit a PR when it is ready.

[sp4ghet]

I tried this in 86c2d22